This invention relates to a sol-gel precursor solution for forming a perovskite ferroelectric material and a method for forming ferroelectric materials for integrated circuits, with particular application for fabrication of ferroelectric dielectric and piezoelectric materials. Particular aspects of the invention relate to lead containing ferroelectric dielectric materials, including lead zirconate titanate (PZT), lead lanthanum zirconate titanate (PLZT), lead magnesium niobate (PMN), lead iron niobate (PFN).
In recent years, the use of the ferroelectric materials for random access memory (RAM) elements has reached commercial applications in the semiconductor industry. Ferroelectric dielectric materials have very high dielectric constants (typically xcex5 greater than 500) providing for memory elements that have high charge storage capacity. Ferroelectric memory elements are non-volatile, consume low power and are programmable with low voltage, e.g. less than 5V. Other advantages include fast access times, ( less than 40 ns), radiation hardness, and robustness with respect to virtually unlimited read and write cycles.
Ferroelectric dielectrics are of also of interest for applications as coupling and decoupling capacitors, and for filter elements operating at low frequency ( less than 1 Hz) up to microwave (GHz) frequencies. The relatively low value of the dielectric constant of conventional dielectrics, typically silicon dioxide and silicon nitride (xcex5 less than 10) limits the capacitance attainable to about 2 to 3 fF/xcexcm2. The high dielectric constant of ferroelectric dielectric materials allows for capacitances greater than 30 fF/xcexcm2. A number of integrated circuit applications would benefit from large on-chip capacitances in the nF range. Consequently, there is much interest in ferroelectric dielectric materials for larger, high value capacitors, as well as for smaller memory elements.
Since ferroelectric materials may also exhibit useful piezoelectric and non-linear optical properties, there is also much interest in providing improved methods for making thin film ferroelectric materials for optoelectronics and other applications, e.g. waveguides, electro-acoustic transducers, surface acoustic wave devices, non-linear optical devices, optical modulators and piezoelectric devices. The high piezoelectric coupling coefficient of ferroelectric materials make them suitable as actuators for micromachine structures incorporated in silicon micro-circuits.
Ferroelectric dielectric materials with large dielectric constants include ferroelectric perovskites, which are complex metal oxides of the general structure ABO3 in which the A and B sites of the perovskite structure are occupied by one or more different metals. Particular perovskite ferroelectric materials which have made the breakthrough in integrated circuit applications include, for example, lead zirconate titanate PbZrxTi1xe2x88x92xO3 (PZT), lead lanthanum zirconium titanate (PLZT), barium titanate (BT), and barium strontium titanate (BST).
PZT has a higher dielectric constant and may be formed at lower temperature than BST. On the other hand, PZT formed by conventional methods shows dispersion, at frequencies above xcx9c100 MHz, above which the dielectric constant drops to a low value. BST has a flat dielectric response up to xcx9c5 GHz and is favoured for high frequency GaAs integrated circuit applications.
The interest in using ferroelectric materials for applications in non-volatile DRAMs has led to rapid development of improved processes for deposition of thin layers of ferroelectric dielectric materials. Known deposition methods which have been investigated include, for example, metallo-organic sol-gel processes, and other spin-on liquid processes based on metallo-organic decomposition, chemical vapour deposition (CVD) and sputtering, laser ablation, electron beam deposition and ion beam deposition.
The integration of ferroelectric materials for capacitor dielectrics for integrated circuits, or for other device structures, requires a process which is compatible with known semiconductor process technologies. Furthermore, the properties of ferroelectric materials provided as thin films are found to differ from bulk ferroelectric materials. In comparison with preparation of bulk ferroelectric materials, factors including film stress, interactions with substrate materials, and restrictions on process temperatures may significantly influence the characteristics of thin films of ferroelectric materials. Thus, much work has been devoted to developing low temperature processes for formation of thin films of ferroelectric dielectrics compatible with semiconductor processing for CMOS, bipolar and bipolar CMOS technologies.
For integrated circuit applications, a preferred known process for forming thin films of ferroelectric materials is based on a technique, generally known as a sol-gel process, in which a complex oxide is prepared from a sol-gel precursor solution comprising a mixture of metallo-organic1 compounds, e.g. alkoxides dissolved in an organic solvent, and/or organic metal salts dissolved in an appropriate solvent e.g. an acid or alcohol. The sol-gel process for preparing metal oxides proceeds by the hydrolysis of a metal organic compound to form a sol comprising metal oxide precursors. This process is well known for forming single component oxide glasses and multi-component oxide glasses from a precursor mixture of metal alkoxides. Formation of metal oxide bonds and growth of metal oxide chains and networks in the solution eventually lead to gelation. The hydrolysis and polymerization by condensation (polycondensation) reactions are controlled by factors such as the amount of water, pH, presence of acid or base catalysts, and reaction sequence, for example, as described in U.S. Statutory invention registration no. H 626, published Apr. 4, 1989, entitled xe2x80x9cSol-Gel Ceramic Oxidesxe2x80x9d to Covino which relates to formation of silicate glasses. In the latter disclosure, it is described how it is known that lowering of pH tends to form oxide networks and chains, forming a polymer network, and leading to gelation without formation of colloidal oxides.
1In the context of sol-gel processing of complex oxide ceramics, the term organo-metallic or metallo-organic has often used to denote metal containing organic compounds used as precursors, including metal alkoxides, metal carboxylates and metal beta diketonates. In organic chemistry, the term xe2x80x9corgano-metallicxe2x80x9d or metallo-organic is more generally used to denote a compound having a metal-carbon bond. 
In a conventional method of sol-gel processing of piezo-electric and ferroelectric dielectric thin films of the general formula ABO3 using a sol-gel precursor solution, a precursor solution is provided comprising metal A as an organo-metallic salt, e.g. a metal acetate, and a mixture of metals B as alkoxides, provided in the required stoichiometric proportions. For example, to make PZT, a precursor mixture of a soluble organic lead salt, e.g. lead acetate tri-hydrate, and a mixture of zirconium propoxide Zr(OC3H7) and a titanium iso-propoxide Ti(OC3H7)4 is dissolved in a suitable solvent e.g. an alcohol, or mixture of solvents. The lead salt is dissolved in a suitable anhydrous solvent such as methoxy-ethanol, and the solution is dehydrated, and then the zirconium and titanium propoxides, also dissolved methoxy-ethanol, are added in stoichiometric ratio to provide the B metal cations.
The metal oxide precursor solution forms a mixture of metallo-organic intermediate compounds which react to form a metal oxide precursor. The viscosity and surface tension of the precursor solution is adjusted to allow a layer with a controlled thickness to be spin-coated or dip-coated onto a substrate, as required, depending on the particular application.
Organic metal oxide precursors other than metal alkoxides which have been reported include metal beta-diketonate (e.g. acetyl acetonate) or metal carboxylates, e.g. acetates. For example U.S. Pat. No. 4,946,710 to Miller entitled xe2x80x9cMethod for preparing PLZT, PZT, and PLT sol-gels and fabricating ferroelectric thin filmsxe2x80x9d, lists precursors including lead acetate, lead tetraethylhexanoate, zirconium, acetyl acetonate, titanium tetra-butoxide butanol complex, titanium iso-propoxide, zirconium tetra-butoxide, lanthanum 2,4, pentadionate, and other acetates and alkoxides which are commercially available and form organo-metallic polymer gels. Nevertheless it is noted that precursor compounds with bulky organic groups are likely to result in porous materials with defects, and thus metal compounds derived from methanol, ethanol, butanol, propanol and acetic acid derived compounds were preferred.
Desirably, there is a common solvent for all precursor solutions of metals A and B. When different solvents are used, solubilization of the metal precursors in different solvents may hinder the formulation of a homogeneous sol-gel solution. In known methods, solvents comprising alcohols, methanol, butanol, propanol, etc. and methoxy-alcohols, e.g. methoxy-ethanol, 1-methoxy-2-propanol are preferred.
In PZT, the ratio of zirconium to titanium occupying the B sites of the ABO3 structure may be varied, and is typically in the range from 20:80 to 80:20, with 40:60 ratio of Zr:Ti being typical. In doped or modified materials, another metal may occupy a proportion of the A sites. One particular example is lead lanthanum zirconium titanate (PLZT) where some of the A sites are filled by lanthanum. PZT may alternatively be modified or doped with other metals, including niobium, tantalum, iron, aluminum and others. These metals are also added in the desired proportion to the sol-gel precursor solution in the form of organo-metallic compounds, typically as metal alkoxides M(OR)x or metal carboxylates, typically a metal acetate. Dopant metals are alternatively added as chlorides of iron, or vanadium, e.g. VCl3, or a chloride or nitrate of lanthanum.
Conventionally, to provide a thin layer of ferroelectric, a substrate is spin-coated or dipped to provide a thin layer on the substrate, and then a heating step at relatively low temperature, xcx9c200xc2x0 C. to 400xc2x0 C., results in pyrolysis, i.e. thermal decomposition of the organo-metallic1 compounds, and drives off solvent and volatile organics, and leaving a layer comprising the mixed metal oxides, which transform at sufficiently high temperature, to form a layer of the ferroelectric mixed oxide. The lower temperature step is often referred to as xe2x80x9cfiringxe2x80x9d, or xe2x80x9cbakingxe2x80x9d. Heating to a higher temperature, typically 600-800xc2x0 C., causes crystallization to a ferroelectric perovskite phase of a mixed oxide having the required functional properties. The higher temperature step may is referred to variously, e.g. as annealing, or sintering. It is known that the nature of the ferroelectric film is sensitive to the substrate, and to the processing conditions.
Bernstein et al. in European Patent Application No. 0 489 519 A2, entitled xe2x80x9cSol-gel processing of piezoelectric and ferroelectric filmsxe2x80x9d, report that rapid localized heating avoids formation of a crust on the surface of the layer, which may prevent outgassing of volatile organic components during heating in a conventional furnace tube. Rapid heating, e.g. by localized heating the substrate on a hot plate, was found to cause localized stabilization of the film while allowing continued outgassing of organics from the surface, a technique that was found to be advantageous in lowering the crystallization temperature. It was also reported that properties of film were more linear with applied voltage when crystallization was carried out in an inert or reducing atmosphere rather than an oxidizing atmosphere.
Bernstein refers to the method of Sayer and Yi described in International Patent Application WO-90/13149, entitled xe2x80x9cSol gel process for preparing Pb(Zr,Ti)O3 thin filmsxe2x80x9d. Sayer used acetic acid as a solvent for both lead acetate trihydrate and zirconium and titanium propoxides, rather than methoxy-ethanol, which is a known teratogen. Sayer and Yi provide a method of preparing crack free thin films by addition of what is called a xe2x80x9cfiring additivexe2x80x9d, e.g. a glycol such as, ethylene glycol, glycerol, tetraethylene glycol, or a polyethylene glycol, for adjusting the viscous state transition temperature of the sol-gel precursor solution. After heating at 300xc2x0 C. to 500xc2x0 C. to pyrolyse the film, crystallization was induced by a lengthy 6 hour anneal at about 600xc2x0 C. However, for integrated circuit applications, a more rapid anneal step is desirable.
In the earlier work of Sayer et al. described in the above mentioned EP Patent Application, the choice of precursor compounds and solvents is important in controlling the characteristics of thin film ferroelectric materials. Precursor compounds should preferably have a high metal content, high solubility in the selected solvent, decompose without evaporating, and be chemically compatible with one another. The solvent must have appropriate boiling point and suitable viscosity and surface tension. Water and/or propanol may be added to adjust the viscosity, and reduce surface tension to increase wettability of the substrate. A chelating agent is added to prevent hydrolysis of the sol-gel solution, and is preferably glacial acetic acid, although other acids may be used.
The order of mixing of the precursor constituents may also be important. For example, Sayer reported that in mixing the lead salt and metal alkoxides, it is important to add zirconium propoxide first because it reacts with acetic acid to form a non hydrolyzable solution which protects the titanium iso-propoxide from hydrolysis. If Ti iso-propoxide is added first, it reacts with the acetic acid to form mono- or di-acetylates and condensation occurs with the formation of poly-titanyl acetylates. In later work discussed in an article entitled xe2x80x9cSol-gel processing of complex oxide filmsxe2x80x9d, in Ceramic Bulletin, vol. 70, no. 7, 1991, pages 1173-1179, Yi and Sayer mixed the Ti and Zr precursors before addition of the lead salt and acetic acid. This procedure has been followed by Schwartz et al. under the name xe2x80x9cinverted mixing ordersxe2x80x9d as reported in Integrated Ferroelectrics vol. 2, 1992, pages 243-254, entitled xe2x80x9cSolution chemistry effects in Pb(Zr,Ti)O3 thin film processingxe2x80x9d.
To provide a homogeneous sol-gel solution, the mixture is agitated, preferably in an ultrasonic tank, until all solids are dissolved. A filtered solution is stable in air, and may be stored in a sealed container.
Sayer et al. also highlight that the low temperature heating step, which they term xe2x80x9cfiringxe2x80x9d, which pyrolyses the organo-metallic compound to an inorganic film, is key to the preparation of the crack free films having desired characteristics including crystal structure, grain size, transparency, and surface roughness.
Evaporation of the solvents and volatile components causes a large volume change, and thus generates internal stresses. During firing, volatile organic components are driven off and the organic film changes to fine mixture of metal oxides, and free carbon. Then at higher temperature the free carbon oxidizes, is released as carbon dioxide, and the mixture of oxides transforms to a transparent amorphous PZT film. Processing and firing under vacuum was found to be advantageous in extracting water uniformly and for uniform decarbonization.
In the method of Sayer et al., it is believed that high boiling point and latent heat of a glycol additive raises the solution evaporation temperature in the first stage towards the melt temperature in the second stage, which retains atom mobility and reduces tendency for cracking.
Other sol-gel based processes for providing improved quality ferroelectric thin films are described in the following references.
Swartz et al. in U.S. Pat. No. 5,198,269 entitled xe2x80x9cProcess for making sol-gel deposited ferroelectric thin films insensitive to their substratesxe2x80x9d, describe a stepwise process for improving the quality of a perovskite ferroelectric thin film by depositing a thin xe2x80x9cinterlayerxe2x80x9d of a first perovskite material, e.g. PbTiO3, or SrTiO3 on which is adherent to the substrate, before depositing a second perovskite material, e.g. PLZT or other ferroelectric material. The interlayer was found to improve crystallinity of the PLZT and provide for deposition on a wider range of substrates.
Maniar in U.S. Pat. No. 5,271,955 and continuation U.S. Pat. No. 5,391,393, both entitled xe2x80x9cMethod for making a semiconductor device having an anhydrous ferroelectric thin filmxe2x80x9d, describes a sol-gel method in which an anhydrous sol-gel precursor is prepared, without hydrolyzing the sol-gel solution. By using anhydrous lead acetate, there is no need to dehydrate the solution, and this compound shows enhanced reactivity with other components. Unlike other known methods in which a condensate is formed by mixing the solutions and then hydrolyzing, this method uses thermally induced condensation, -i.e. by boiling (or refluxing) to induce a heterogeneous condensation reaction between metal precursors in solution. Preparation of the anhydrous solution in an oxygen containing dry ambient resulted in a more stable mixture, reduced degradation of the solution by atmospheric humidity and increased shelf life. Maniar notes that exclusion of water avoids bulky precursor molecules with a high degree of internal strain which occurs with hydrolyzed precursors that tend to polymerize preferentially with a single metal element. Films were prepared from the anhydrous sol-gel solution in a conventional manner, with a heating step to drive off solvent and organic ligands, followed by sintering to interdiffuse metals and form perovskite thin film.
A long shelf life of anhydrous sol-gel solution was reported, using excess lead from 0 to 20%. Excess lead is known to suppress formation of an intermediate pyrochlore phase during annealing. The formulation of the sol-gel exclusively by thermal condensation and in the absence of hydrolysis yields an anhydrous amorphous sol-gel having a uniform condensate composition. The method provides improved durability and lower temperature conversion to perovskite crystalline phase.
Mackenzie et al. in U.S. Pat. No. 5,342,648, entitled xe2x80x9cMethod for forming amorphous ferroelectric materialsxe2x80x9d discusses how the morphology of polycrystalline thin films dictates characteristics of the material, and suggests growing single crystal films by sol-gel techniques to avoid shortcomings introduced by grain boundaries in polycrystalline films. Mackenzie uses a precursor mixture of metal alkoxides dissolved in alcohol such as absolute ethanol to provide an anhydrous mixture which is not reacted with water until it is coated onto the substrate. PZT is prepared from a mixture of Ti propoxide and Zr propoxide dissolved in propanol, mixed with lead acetate dissolved in propanol. Hydrolysis and polycondensation of this mixture occurs in situ in the thin film and provides amorphous thin films. That is, when water, e.g. water vapour in air reacts with the thin film, and by control of humidity during processing, a polycondensation of a polymer having metal oxygen-metal bonds occurs controllably. The alkyl groups are released as corresponding alcohol. A pre-polymer may occur as particles in the gel.
Teeowee et al., in U.S. Pat. No. 5,384,294 entitled xe2x80x9cSol-gel derived lead oxide containing ceramicsxe2x80x9d report a method for production of PZT using a mixture of a lead carboxylate (e.g. lead acetate) in alcohol, and other metal cations provided as a mixture of alkoxides and alkanolamines, i.e. amine derivatives of the more commonly used alkoxides. These are prepared by reacting metal alkoxides with an amine. The alkanolamines are less reactive and less hygroscopic with improved solubility in higher alcohols. Shelf life is prolonged relative to unmodified metal alkoxides. PVP (polyvinyl pyrollidine) is added for sol rheology control, i.e. to control the viscosity and flow properties of the sol-gel solution. Thin films with exceptionally high dielectric constants, up to 3000, were obtained.
A number of processing difficulties arise in known processes including lack of batch to batch uniformity and reproducibility due to instability and degradation of the sol-gel precursor solutions. Special precautions are required for making and storing anhydrous solutions to keep out atmospheric moisture.
Non-uniformities in coating may occur due to inadequate control of the viscosity or surface tension of organic solutions. Cracking may occur during the heating phase, as a result of stress and macroscopic defects generated by the large volume changes when volatile organic components and solvents are driven off. An excessive volume ratio of organic products to the inorganic polymer network can cause porosity and cracks in the fired films, may inhibit reaction of the precursor film components, and give rise to poor crystallization in the fired film. Stress, or poor adhesion to the substrate then may result in delamination of films. Stress and cracking are exacerbated in thicker films ( greater than 1 xcexcm).
While it has been reported that cracking of films may be controlled by firing at lower temperature for extended periods, (i.e. initial heating stage in excess of 20 minutes), extended thermal processing may not be compatible with integrated circuit fabrication, or may result in low quality crystalline layers, with poor ferroelectric characteristics. Rapid thermal processing is preferable for integrated circuit fabrication. However, rapidity of reaction may exacerbate any inhomogeneities in the film, and generation of stresses are significantly. different in rapid thermal processing as compared with furnace annealing.
As in all semiconductor processing, high purity starting reactants are desirable. Nevertheless, because organic metal precursors are used, carbon containing residues of the volatile organic precursors may remain in the film, either in the bulk, or trapped at boundaries between grains. Oxygen loss from the structure may occur during oxidation of residual carbon to carbon dioxide, which results in an oxygen deficient stoichiometry of the ferroelectric phase.
Oxygen stoichiometry is important because the functional properties of the ferroelectric oxide are strongly dependent on the xe2x80x9coxygen octahedraxe2x80x9d in the crystal structure. A minimum oxygen stoichiometry is required to maintain the non-centro symmetry of the unit cell. Under most circumstances, known chemical processes yield an oxygen stoichiometry above the critical limit to form a desired crystal structure, and the film is identifiable as a ferroelectric. Nevertheless, oxygen non-stoichiometry, remains a major difficulty to be overcome. If oxygen stoichiometry occurs non-uniformly throughout the film, the average properties of the material will be degraded. For example, processed ferroelectric films generally show dielectric constants which are lower and coercive fields for polarization reversal which are higher than those observed in the bulk material. Films are often not able to withstand repeated polarization reversals for as many repetitions as required. Thus, removal of carbon from the as-coated films is identified as a critical processing stage, and known methods suggest that removal of carbon is best achieved by firing (heating) the films at 350xc2x0 to 400xc2x0 C. for a few minutes. Apart from organic precursors, and organic solvents, other organics added as firing additives or to adjust surface tension and viscosity also add to the organic loading of the solution. In lead containing ferroelectrics, the organic lead salt is usually added in excess to suppress formation of a pyrochlore phase. Thus, each of these components contributes to an excess of carbon, and thus may adversely affect oxygen stoichiometry.
Thus, the present invention seeks to provide a sol-gel precursor solution for formation of a ferroelectric material for integrated circuit applications, and a method of fabrication of ferroelectric dielectric or piezoelectric materials, which avoids or reduces the above mentioned problems, and with particular application to a sol-gel precursor solution and a method for forming lead containing perovskite ferroelectric materials.
Thus according to one aspect of the present invention there is provided a sol-gel precursor solution for preparing a perovskite ferroelectric material of general formula ABO3 wherein A and B each comprise one or more metals, the solution comprising a mixture of compounds of metals A and B wherein a least one of the compounds of A and B is provided by a solution of soluble inorganic metal salt, and the other metals are provided as metallo-organic compounds.
The sol-gel precursor solution comprises an inorganic metal precursor compound of at least one of the metals of the ferroelectric oxide. Preferably these are inorganic metal salts provided in an aqueous solution, and other constituents are provided in solvents that are water miscible. Thus the proportion of organic components of the precursor solution is reduced relative to known precursor solutions in which all metals are provided by metallo-organic precursor compounds.
According to another aspect of the present invention there is provided a sol-gel precursor solution for preparing a ferroelectric dielectric for an integrated circuit comprising a complex oxide the general formula of ABO3, where A and B each comprise at least one metal, the sol-gel precursor solution comprising a mixture of a first constituent and a second constituent,
the first constituent comprising a solution of an inorganic salt of at least one of metals A in an aqueous solution,
the second constituent comprising a solution of an organo-metallic compound of at least one other metal B selected from the group consisting of metal alkoxides, metal alkoxide derivatives, metal carboxylates and metal betadiketonates, dissolved in a water miscible solvent selected from the group consisting of alcohols, carboxylic acids, and ketones,
the first and second constituents being mixed to form a homogeneous sol-gel solution.
The combination of inorganic and organic precursors results in a reduction of organic by-products, and thereby aids in reducing the carbon content in the bulk of the film during the spinning and firing stages of film processing. The time required for firing and annealing cycles is substantially reduced. The oxygen stoichiometry of the film material, the surface morphology, transparency, thickness uniformity, film stress, and the electrical properties of the ferroelectric film are also improved.
The sol-gel precursor solution provides particular benefits for processing for integrated circuit applications using rapid thermal annealing at relatively low temperatures compatible with fabrication of integrated circuits. In particular, a suitable choice of annealing atmosphere and processing conditions provides for improved quality of ferroectric materials.
Thus, according to another aspect of the present invention there is provided a method of providing a layer of a perovskite ferroelectric material of structure ABO3 on a substrate comprising:
providing on the substrate a coating of a sol-gel precursor solution a sol-gel precursor solution for preparing a perovskite ferroelectric material of general formula ABO3 wherein A and B each comprise one or more metals, the solution comprising a mixture of compounds of metals A and B wherein a least one of the compounds of A and B is provided by a solution of a soluble inorganic metal salt, and the other metals are provided as metallo-organic compounds,
heating the coating to stabilize and drive off volatile organic components and solvent and to pyrolise the coating to form oxides, and then
heating the coating to a higher temperature to form a required crystalline phase of a ferroelectric material,
the heating steps being carried out by rapid thermal processing in an annealing atmosphere comprising oxygen, ozone and water vapour.
Preferably both steps of heating the coating are carried out by rapid thermal annealing. The presence of ozone and water vapour during annealing was found to speed up oxidation and provide for crystallization at lower temperature.
Beneficially, a sol-gel precursor solution is provided for forming lead containing perovskite type mixed oxides using an inorganic lead precursor for metal A. For example, lead zirconate titanate is prepared from a mixture in which lead is provided as a soluble inorganic lead salt, preferably lead nitrate. Zirconium and titanium may be provided as alkoxides, or alternatively zirconium may be provided an inorganic salt, e.g. zirconium nitrate. Other soluble inorganic metal salts, e.g. chlorates, may be used if they are not incompatible with device processing. These sol-gel solutions were also found to be stable during storage over extended periods. Dopant metals may be added to the sol-gel precursor solutions in appropriate quantities by addition for example, of organic or inorganic compounds of niobium, lanthanum, aluminum, vanadium, tantalum or chromium.
Other lead containing perovskite ferroelectric materials are provided by suitable mixtures or inorganic and organic metal compounds, e.g. lead iron niobate, and lead magnesium niobate, lead strontium niobate, and lead strontium titanate. The latter materials in particular have useful piezoelectric properties.
Similarly, a sol-gel precursor solution for forming barium strontium titanate may also be prepared in which strontium is provided by a water soluble inorganic strontium salt, and barium and titanium are provided as a barium carboxylate and a titanium alkoxide.
According to another aspect of the present invention there is provided a method of providing a layer of a perovskite ferroelectric material of structure ABO3 on a substrate, comprising:
providing a sol-gel precursor solution comprising mixture of an inorganic metal salt of a metal A , and an alkoxide of at least one metal B in predetermined proportions dissolved in a aqueous solvent,
providing a coating of the sol-gel precursor solution on the substrate;
heating the coating to stabilize and drive off volatile organic components and solvent and to pyrolise the coating to form oxides, and then
heating the coating to a higher temperature to form a required crystalline phase of a ferroelectric material.
Thus, the sol-gel precursor solution is conveniently provided as an aqueous, solution. Beneficially, the solution is prepared having a suitable viscosity and surface tension, to allow for spin-coating onto a substrate using conventional known coating apparatus.
The method may be used advantageously in fabricating films for devices structures with reduce film cracking during the thermal processing on large diameter substrate wafers. Stress reduction is particularly advantageous in formation of thicker films, i.e. xcx9c10 xcexcm thick, which may be achieved by multiple coatings.
The steps of coating with precursor solution and heating may be repeated sequentially, as required, to build up a layer of a required thickness of ferroelectric material. Thicker structures are provided by building up multiple coatings, layer by layer.
Preferably the resulting film is annealed at between 450xc2x0 C. to 700xc2x0 C. to induce crystallization of the film to form a crystalline perovskite phase. Annealing by rapid thermal annealing is preferred, particularly for integrated circuit applications. Advantageously, annealing by carrying out each heating steps in oxygen and ozone in the presence of water vapour provides for crystallization at reduced temperature, and with reduced processing times relative to annealing in dry oxygen.
Thus, there is provided a low temperature process by which perovskite ferroelectrics may be reproducibly deposited on an appropriate substrate, and compatible with fabrication of integrated circuits by conventional CMOS and bipolar process technologies.
Preferably, high purity precursor materials are selected to avoid constraints on processing which may arise from other factors, i.e. contamination of semiconductor devices structures by certain elements which may be detrimental to device characteristics. These elements are desirably eliminated from processes for semiconductor processing, e g. a chloride free process may be preferred for silicon processing technology.
In application of the sol-gel solution in a method for fabrication of PZT, a wide range of compositions from about 20:80 to 80:20 zirconium to titanium ratio was found to produce ferroelectric dielectrics with excellent characteristics over a wide range of frequencies, up to microwave frequencies. A preferred composition was 60:40 Zr:Ti. Dopants, i.e. lanthanum or niobium, are added to the PZT precursor if desired, and did not significantly affect the microwave performance.
The sol-gel precursor mixture and method was also used successfully in fabrication of other lead containing complex oxides having suitable properties for ferroelectric dielectrics and as piezoelectric materials. For example, good quality PLZT, and lead magnesium niobate and lead iron niobate thin layers were also provided using one or more appropriate inorganic metal precursor compounds.
According to a further aspect of the present invention there is provided, a method of forming a ferroelectric dielectric comprising polycrystalline lead zirconate titanate for integrated circuit applications, comprising:
providing on a substrate a coating comprising an aqueous sol-gel precursor solution comprising a mixture of lead nitrate, and zirconium and titanium alkoxides;
heating the coating to form an amorphous layer of ferroelectric precursor material;
then, annealing the layer of amorphous ferroelectric precursor material at a temperature sufficient to cause a phase transformation to a ferroelectric polycrystalline perovskite phase;
both the step of heating and the annealing step comprising heating in an oxygen containing atmosphere in the presence of water vapour, thereby providing a ferroelectric layer of lead zirconate titanate.
In providing materials, including PZT, for high frequency (microwave) applications, annealing is beneficially carried out in an atmosphere comprising oxygen and ozone, and in the presence water vapour, at lower temperature, i.e. xcx9c500xc2x0 C. Under these conditions uniform growth of fine grained polycrystalline material occurs, and grain sizes growth above about 20 nm were not observed. Superior high frequency characteristics were observed for material characterized by uniform small grain sizes.
Thus the present invention provides a sol-gel precursor solution for formation of ferroelectric dielectric material and piezoelectric materials, and a method of formation of a ferroelectric material which overcome or reduce some of the above mentioned problems, with particular application to forming lead containing perovskite ferroelectric materials including PZT, PLZT and PMN and others.
The invention provides a sol-gel precursor solution and a method for preparing a thin layer of a perovskite ferroelectric material of general formula ABO3 wherein A and B each comprise one or more metals, with particular application for integrated circuit applications and for fabrication of microelectronic devices. Whereas all known sol-gel precursor solutions for forming complex ferroelectric metal oxides use organic metal compounds as precursors for the metal oxide, in this method, at least one of the main constituent metals in the precursor solution is provided as a soluble inorganic salt. In particular, for forming lead containing perovskite materials, lead is derived from an inorganic lead precursor, and the latter is preferably provided in an aqueous solution.
For example, to form the ferroelectric material lead zirconate titanate according to a first embodiment of the present invention, the metal occupying the A sites in the ABO3 structure comprises lead, and metal occupying the B sites is a mixture of zirconium and titanium, in desired stoichiometric proportions, typically in the range 20:80 to 80:20 Zr:Ti.
A method of preparing a sol gel precursor solution according to a first embodiment which specific to the fabrication of PZT is described below. The following general description of the method also applies to formation of other perovskite ferroelectric materials using other appropriate metal precursors.
A sol-gel precursor solution is provided, resulting from a mixture of metal salts of A and metal salts of B wherein a least one of metal salts of A and B is provided by an aqueous solution of water soluble inorganic metal salt, and the remaining metals are provided as metallo-organic salts of metals A, B, or a mixtures of metallo-organic salts of metals A and B. The aqueous precursor solution including inorganic metal salts thus differs from known prior art sol-gel precursors in which all constituent metals are provided as organo-metallic salts, e.g. as metal alkoxides.
In most respects the preparation of the sol-gel precursor solution, and choice of suitable solvents, control of surface tension and viscosity is similar to that of known sol-gel processes using organo-metallic precursors. The process of the invention differs in that the inorganic metal salts are preferably provided as aqueous solutions of suitably soluble inorganic metal salts, and solvents for the other metal precursors, which are provided as alkoxides or organo-metallic salts are dissolved in a solvent miscible with the aqueous inorganic metal salt. The constituents of the solution are mixed to form a homogeneous solution, preferably with ultrasonic agitation. The organic loading of the precursor solution is thus substantially reduced in respect of the solvents as well as the metal precursors.
In a method of preparing thin layers of ferroelectric material, the sol-gel precursor solution is dip-coated or spin-coated on to a suitably prepared substrate in a conventional manner and heat treated. As in known sol-gel processes, a low temperature heating step. For example, for PZT, this heating step is typically in the range to about 400xc2x0 C. or below. This low temperature step stabilizes the film and drives off volatile components and solvent from the precursor mixture leaving a mixed oxide precursor. Then a higher temperature heating step, or annealing, in a suitable atmosphere, transforms the film into the required perovskite ferroelectric phase of the complex mixed oxide. The annealing atmosphere for the lower temperature heating step and the higher temperature heating step is important in determining the properties of the resulting ferroelectric material. As an initial step, the low temperature heating step may be preceded by heating to about 100xc2x0 C. to dry the film, i.e. a drying step similar to that described in U.S. Pat. No. 4,963,390 to Lipeles et al.
As is conventional in spin coating, thicker layers are built up by sequential multiple thin coatings and low temperature heat treatments. Once the required thickness is built up, the layer is annealed at the higher temperature in a suitable atmosphere.
A sol-gel precursor solution for preparing a ferroelectric dielectric for an integrated circuit comprising a complex oxide the general formula of ABO3 where A and B each comprise at least one metal or mixtures of metals is prepared from a mixture of a first constituent comprising an aqueous solution of the inorganic salt of metal A, and a second constituent comprising a solution comprising an organo-metallic salt of at least one other metal B selected from the group of metal alkoxides and metal alkoxide derivative, dissolved in a suitable solvent selected from anhydrous alcohols, acetic acid, acetyl acetone, etc. The constituents are mixed to form a homogeneous sol-gel solution. Adjustment of the pH provides that hydrolysis and polycondensation reactions to form the precursor metal oxide polymer networks are controlled, and if required a viscosity modifier and surface tension modifier are added to provide a solution with the desired rheological properties for spin or dip coating thin or thick layers, as required.
In summary, the general method of the invention for preparing a ferroelectric thin film comprises the following steps:
1. A sol-gel precursor solution is prepared from an inorganic metal salt which is dissolved in a solvent, preferably water, and is mixed with organo-metallic starting material dissolved in the same or a different solvent. When two or more inorganic salts and/or two or more organic solutions are present, mixing is done in a manner so as to avoid the formation of any precipitate;
2. The resulting sol-gel precursor solution is used to make a thin layer on the substrate by use of conventional spray coating, dip-coat or spin-cast methods;
3. The layer is optionally dried by heating to about 100xc2x0 C., and then heated to 350-400xc2x0 C. for a time ranging from a few seconds to a few minutes, preferably by rapid thermal processing, to remove the volatile solvents and carbon, and to form a stress-free amorphous film containing the inorganic metal oxide constituents;
4. A rapid thermal annealing treatment, preferably in an oxygen containing atmosphere, in the form of a 100xc2x0 C./sec ramp to above 450xc2x0 C., typically above 500xc2x0 C. and 600-800xc2x0 C. if compatible with other process steps, which is then held for xcx9c10 seconds to several minutes, to crystallize the films into the required crystallographic phase.
Advantageously, it was found that the use of an inorganic metal salt reduced the organic components of the precursor solution, reducing the anneal time, and allowing for transformation to a ferroelectric phase to occur at lower temperature, as will be described in the following embodiments. The resulting films had lower carbon content, improved oxygen stoichiometry and reduced stress. These improvements are attributed to the reduction in organic residues in the precursor films. Use of the sol-gel precursor in combination with rapid thermal processing and annealing, preferably in an oxygen containing atmosphere in the presence of water vapour, was beneficial in providing ferroelectric materials of improved quality for integrated circuit applications.
The reduction in organic loading of the precursor solution, increases the availability of oxygen during processing and thus improves the quality of the fired films.
Inorganic compounds may often be obtained with higher purity than organic precursor materials, and at lower cost. During processing the inorganic salt, e.g. a nitrate, may readily decompose to provide oxygen to the reaction which assists in maintaining high level of oxygen stoichiometry.